1. Field of the Invention
The invention relates to electro-optical readers or scanning systems, such as bar code symbol scanners, and more particularly to retro-reflective laser scanning modules for use in applications requiring particularly compact scanners.
2. Description of the Related Art
Electro-optical readers, such as bar code symbol readers, are now quite common. Typically, a bar code symbol comprises one or more rows of light and dark regions, typically in the form of rectangles. The widths of the dark regions, i.e., the bars, and/or the widths of the light regions, i.e., the spaces, between the bars indicate encoded information to be read.
A bar code symbol reader illuminates the symbol and senses light reflected from the coded regions to detect the widths and spacings of the coded regions and derive the encoded information. Bar code reading type data input systems improve the efficiency and accuracy of data input for a wide variety of applications. The ease of data input in such systems facilitates more frequent and detailed data input, for example to provide efficient inventories, tracking of work in progress, etc. To achieve these advantages, however, users or employees must be willing to consistently use the readers. The readers therefore must be easy and convenient to operate.
A variety of scanning systems are known. One particularly advantageous type of reader is an optical scanner which scans a beam of light, such as a laser beam, across the symbols. Laser scanner systems and components of the type exemplified by U.S. Pat. Nos. 4,387,297 and 4,760,248 xe2x80x94which are owned by the assignee of the instant invention and are incorporated by reference hereinxe2x80x94have generally been designed to read indicia having parts of different light reflectivity, i.e., bar code symbols, particularly of the Universal Product Code (UPC) type, at a certain working range or reading distance from a hand-held or stationary scanner.
FIG. 1a illustrates an example of a prior art bar code symbol reader 10 implemented as a gun shaped device, having a pistol-grip type of handle 53. A lightweight plastic housing 55 contains a light source 46, a detector 58, optics 57, signal processing circuitry 63, a programmed microprocessor 40, and a power source or battery 62. A light-transmissive window 56 at the front end of the housing 55 allows an outgoing light beam 51 to exit and an incoming reflected light 52 to enter. A user aims the reader at a bar code symbol 70 from a position in which the reader 10 is spaced from the symbol, i.e. not touching the symbol or moving across the symbol.
As further depicted in FIG. 1a, the optics may include a suitable lens 57 (or multiple lens system) to focus the scanned beam into a scanning spot at an appropriate reference plane. The light source 46, such as a semiconductor laser diode, introduces a light beam into an optical axis of the lens 57, and the beam passes through a partially-silvered mirror 47 and other lenses or beam shaping structures as needed. The beam is reflected from an oscillating mirror 59 which is coupled to a scanning drive motor 60 energized when a trigger 54 is manually pulled. The oscillation of the mirror 59 causes the outgoing beam 51 to scan back and forth in a desired pattern.
A variety of mirror and motor configurations can be used to move the beam in a desired scanning pattern. For example, U.S. Pat. No. 4,251,798 discloses a rotating polygon having a planar mirror at each side, each mirror tracing a scan line across the symbol. U.S. Pat. Nos. 4,387,297 and 4,409,470 both employ a planar mirror which is repetitively and reciprocally driven in alternate circumferential directions about a drive shaft on which the mirror is mounted. U.S. Pat. No. 4,816,660 discloses a multi-mirror construction composed of a generally concave mirror portion and a generally planar mirror portion. The multi-mirror construction is repetitively reciprocally driven in alternative circumferential directions about a drive shaft on which the multi-mirror construction is mounted.
The light 52 reflected back by the symbol 70 passes back through the window 56 for transmission to the detector 58. In the exemplary reader shown in FIG. 1a, the reflected light reflects off of mirror 59 and partially-silvered mirror 47 and impinges on the light sensitive detector 58. The detector 58 produces an analog signal proportional to the intensity of the reflected light 52.
The signal processing circuitry includes a digitizer 63 mounted on a printed circuit board 61. The digitizer processes the analog signal from detector 58 to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars. The digitizer serves as an edge detector or wave shaper circuit, and a threshold value set by the digitizer determines what points of the analog signal represent bar edges. The pulse signal from the digitizer 63 is applied to a decoder, typically incorporated in the programmed microprocessor 40 which will also have associated program memory and random access data memory. The microprocessor decoder 40 first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyses the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard to which the scanned symbol conforms. This recognition of the standard is typically referred to as auto discrimination.
To scan the symbol 70, the user aims the bar code reader 10 and operates movable trigger switch 54 to activate the light source 46, the scanning motor 60 and the signal processing circuitry. If the scanning light beam 51 is visible, the operator can see a scan pattern on the surface on which the symbol appears and adjust aiming of the reader 10 accordingly. If the light beam 51 produced by the source 46 is marginally visible, an aiming light may be included. The aiming light, if needed produces a visible-light spot which may be fixed, or scanned just like the laser beam 51. The user employs this visible light to aim the reader at the symbol before pulling the trigger.
The reader 10 may also function as a portable data collection terminal. If so, the reader would include a keyboard 48 and a display 49, such as described in the previously noted U.S. Pat. No. 4,409,470.
In electro-optical scanners of the type discussed above, the xe2x80x9cscan enginexe2x80x9d including the laser source, the optics the mirror structure, the drive to oscillate the mirror structure, the photodetector, and the associated signal processing and decoding circuitry all add size and weight to the scanner. In applications involving protracted use, a large heavy hand-held scanner can produce user fatigue. When use of the scanner produces fatigue or is in some other way inconvenient, the user is reluctant to operate the scanner. Any reluctance to consistently use the scanner defeats the data gathering purposes for which such bar code systems are intended. Also, a need exists for compact scanners to fit into small compact devices, such as notebooks or palm size computers.
Thus, an ongoing objective of bar code reader development is to miniaturize the reader as much as possible, and a need still exists to further reduce the size and weight of the scan engine and to provide a particularly convenient to use scanner. The mass of the moving components should be as low as possible to minimize the power required to produce the scanning movement.
It is also desirable to modularize the scan engine so that a particular module can be used in a variety of different scanners. A need exists, however, to develop a particularly compact, lightweight module which contains all the necessary scanner components.
Smaller size scanning components tend to operate at higher scanning frequencies. In typical bar code scanning applications, however, the scanning frequency of the moving beam spot should be relatively low, typically 20 Hz or less. If the frequency increases, the speed of the spot as it passes over the symbol increases. The signals produced by the detector also increase in frequency, and consequently the bandwidth of the processing circuitry for analyzing the detector signals must be increased. Also, operation at higher scanning frequencies generally produces detector signals which include higher levels of noise, making accurate decoding more difficult.
The objective of this invention is to develop an entirely self-contained, electro-optical, retro-reflective scanning module, including all components necessary to generate the light beam, scan the beam in a pattern across a symbol, detect light reflected back by the symbol and process signals representative of the reflected light. In this regard, the retro-reflective module should be small, lightweight and easy to incorporate into a variety of different types of electro optical scanning systems.
Another objective of this invention is to minimize the size and weight of the components used to produce the scanning motion of the light beam, and to collect the reflected light.
Another related objective is to develop an electro-optical scanning system which is smaller and lighter in weight, when held by an operator, and which it is easier to manipulate to scan encoded indicia, as compared to known system.
Another relate objective is to develop an electro-optical scanning module which is not susceptible to specular reflections coming from the exit window of a bar code scanning system.
In keeping with these objects, one feature of this invention is embodied in a self-contained, electro-optical, retro-reflective scanning module for reading optically encoded indicia having parts of different light reflectivity. The module includes a base; a light source on the base, for emitting a light beam along an outgoing path toward the indicia for reflection therefrom; a light detector is mounted on the base, for detecting light reflected from the indicia along an incoming path over a field of view, and for generating electrical signals corresponding to the indicia parts of different light reflectivity; a movable reflector mounted in the paths of the light beam and the reflected light; and a drive on the base, for moving the movable reflector to sweep the light beam across the indicia, and to simultaneously sweep the field of view.
In accordance with this invention, the light source is mounted in a casing, and a stationary collecting mirror is fixedly mounted on the casing, and a stationary fold/scan mirror is fixedly mounted on the collecting mirror. Advantageously, the collecting mirror is concavely curved, and the fold/scan mirror is planar. Also, a bracket holds the mirrors at a distance from the casing.
Another advantageous aspect of this invention is embodied in fixedly mounting the casing and the mirrors at one of two end regions of the base, and in mounting the movable reflector for movement at the other of the end regions of the base. The light detector is mounted at a central region of the base and faces the collecting mirror, and is located at the center of curvature thereof.
The drive includes an elongated support having opposite end portions. The reflector is mounted at one of the end portions of the support. The drive includes a permanent magnet mounted at the other of the end portions of the support, and an electromagnetic coil mounted in proximity to the permanent magnet and operative, when an alternating drive signal is applied to the coil, for producing an alternating magnetic field which acts on the permanent magnet to oscillate the magnet and, in turn, the support and the scanning reflector mounted thereon about an axis located approximately midway between the reflector and the permanent magnet.
In another embodiment, the retro-reflective module includes a folding mirror which is located behind a collection mirror. The folding mirror redirects the laser beam, generated by the focused laser diode assembly, onto an oscillating scanning mirror. In order for the folded laser beam to reach the oscillating scanning mirror, the beam must pass through an aperture in the collection mirror. Positioning of the photodetector and the collection mirror introduces design tradeoffs which impact the overall dimension of the scanning module.
The resulting compact construction enables the retro-reflective module to be placed within a variety of system configurations, especially miniature ones.
Various ways are disclosed for minimizing and eliminating the detection of specular reflection from a scanner""s exit window.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. Further features of the invention are set out in the appended independent claims, and further preferred features are set out in the dependent claims.